this is Ryan from Motonation, the US distributors of Sidi. The place you are going to want to go is linked below. If you visit the site you will notice this guy specializes in resoling off road type boots. He does all of our warranty sole repair and the only guy that Motonation stands behind his work. You will also notice on the site, no matter the boot, he only uses Sidi soles.http://www.factoryresole.com/
as for the warranty, I am not 100% sure. But I will ask. All Sidi boots are covered by a 2 year warranty by Motonation. The lifetime warranty by gore-tex though is not something I am completely familiar with...well the details at least. They are pretty pecular and strict in the rules on their warranty as well as the production. Gore-tex Sidi boots must be built with leather approved by gore-tex vs. the Rain models which are constructed of Lorica.
The construction and design of the boots must be approved by gore-tex as well, so I am sure the stitching of the soles plays into that some how. I dont know if this would be the case, but I have also seen resole jobs where the mold a new sole onto the stitched sole instead of unstitching the boot and restitching a new sole back on. If that was the case, I would see no reason why they would not be covered under the lifetime warranty after the repair since the construction actually wasnt compromised at all.
I will find the answer and get back to you as soon as I know.http://www.factoryresole.com/